"I would like the next president to support a budget that allows us to accomplish the mission that we are asked to perform, whatever that mission may be," Kelly wrote.

While NASA's budget for 2016 is $19.3 billion, the amount of money the organization gets fluctuates from year to year, depending on the amount the president requests for NASA and the amount that Congress actually approves.

When you don't know what kind of budget to expect from year to year, it makes long-term planning difficult. And when you're tasked with building and launching spacecraft millions or even billions of miles from Earth, on missions that take several years, long-term planning is pretty critical.

For NASA's work to be worth it, the funding needs to be there for the duration of each mission — or all the time, money, and research that goes into a given space exploration mission can end up being all for nothing.

NASA's budget peaked during the Apollo era at about 4.4% of the federal budget. But by the end of the 70s, it had dropped to well below 1%. And since 2010, it has hovered around or just below 0.5% of the federal budget.

NASA's 2016 budget was actually higher than expected, which is great news for the agency. But next year the budget could drop again, which could trigger a mad scramble to reshuffle priorities. More programs could end up stalled in the water or canceled all together.

Further, when a new president is inaugurated next year, he or she might direct NASA toward an entirely different mission. That means all of NASA's work on a manned mission to Mars could get shelved until a new president comes along who agrees that we should go there.

Some have proposed that NASA's funding should be set in 5-year or 10-year budgets instead of yearly budgets to ensure the organization can see projects through from start to finish. But since NASA's funding is part of the yearly federal budget, it would be extremely difficult to make that happen.